發(fā)布時間：2018-06-07 15:56

  本文選題：石墨烯 + 石墨炔　； 參考：《山東大學》2016年博士論文

【摘要】：隨著電子器件小型化的不斷發(fā)展,傳統(tǒng)的硅基材料集成電路的尺寸在不斷地縮小,根據(jù)著名的摩爾定律的預測,硅基器件的尺寸將進一步的縮小到1～2nm左右,這個尺寸的大小已經(jīng)進入到分子和原子領域,因此能否利用單個分子來形成納米回路中元器件的這個想法很自然的成為人們非常關注的問題。令人們感到高興的事,單個分子確實可以小型化傳統(tǒng)的硅基微電子器件,并且還可以表現(xiàn)出傳統(tǒng)硅基器件所具有的功能。因此,我們把研究利用單個原子或者利用單個分子來構建電子回路中的功能性元件的方法叫做分子電子學。一直以來,人們都在設計尋找并且制備分子尺寸的電子器件。1959年,美國的物理學家費曼在物理學會年會上發(fā)表了著名的演講"Plenty of Room at the Bottom ",提出了在分子和原子的尺寸上建立電子器件然后構成集成電路的想法,這個想法在當時引起了深遠的影響。傳統(tǒng)的想法是建立好宏觀系統(tǒng)和元件,然后將他們的尺寸縮小,而費曼的想法是直接從原子尺度上來建立電子器件,最后利用這個電子器件組合成電路。相比于傳統(tǒng)的硅基半導體材料來說,應用分子作為電子器件的元件具有許多巨大的優(yōu)勢,例如：①大小尺寸方面：分子的體積小(1-10nm)并且具有非常高的集成密度,在能源消耗及利用效率方面具有極大的優(yōu)勢；②運算速度方面：電子輸運性質較好的分子線可以減小晶體管的渡越時間,減少運算時間；③組裝和識別方面：可以利用特殊的分子間相互作用形成納米尺度自組裝結構。分子識別可以用來調節(jié)電子行為,提供單分子尺度上的開關和感知功能；④合成可修正性：選擇不同的組合和結構就能改變分子電子輸運性質；⑤具有新的功能：某些特殊種類的分子,存在不同的穩(wěn)定結構或者同分異構體,這在傳統(tǒng)固態(tài)材料中是不可能實現(xiàn)的。隨著實驗技術手段的發(fā)展,多種分子電子器件都可以制備出來,這都要歸功于這些先進的設備儀器,這里我們簡單的介紹幾種。①掃描電子顯微鏡技術(STM),是目前最通用的制備原子尺度接觸的方法之一。實驗方法是將含有目標分子的溶液放在金屬的襯底上,利用STM探頭在金屬襯底表面做伸縮運動,從而形成了探頭、目標分子和金屬襯底的三明治電極結構,從而測得電導數(shù)值。此外還有②原子力顯微鏡技術,③透射電鏡技術(TEM)④力學可控破缺結等等。隨著實驗手段的發(fā)展,理論方法也在不斷地創(chuàng)新,用來更精確的表述分子電子器件中的電荷或自旋輸運問題。就目前來說,結合密度泛函理論和非平衡格林函數(shù)方法是處理分子電子輸運的首選方法,并且已經(jīng)廣泛應用于相關的研究中,例如單分子團簇,分子線,石墨烯以及納米管等。自從2004年,英國曼徹斯特大學的Novoselov和Geim首次在實驗上分離出單層石墨烯以來,石墨烯的相關研究就受到了人們的廣泛關注。也就是在這個時期,二維的平面材料成為人們研究的熱點。基于單層的石墨烯,沿著兩個不同的方向進行裁剪,可以得到兩種不同種類的石墨烯納米條帶。通常按照邊緣的樣式,可以將石墨烯納米條帶區(qū)分為鋸齒形石墨烯納米條帶(ZGNRs)和扶手椅形石墨烯納米條帶(AGNRs)。其中,關于ZGNRs的研究更是非常的興盛,先前的研究人員的工作重點是研究ZGNRs的電子輸運特性,并且是將ZGNRs的一部分鏈接到ZGNRs電極上,在這種情況下,就會忽略掉ZGNRs片段與ZGNRs電極的接觸位置效應。再者,ZGNRs具有邊緣態(tài),能夠在Fermi能級處產(chǎn)生較強的電子密度分布,這種奇特電子密度分布可能會對基于ZGNRs電子器件產(chǎn)生特殊的輸運特性。因此基于ZGNRs的研究的非常重要的。人們在研究sp2雜化的石墨烯的同時,也在不斷地探索具有不同雜化形式新的碳的同素異形體。由于石墨炔具有獨特的sp2和sp雜化特性,近幾年石墨炔方面的研究同樣受到廣泛關注,Hirsch認為對石墨炔研究的新時代即將到來。盡管科學家在有關石墨炔結構、合成、特性等方面開展了許多研究工作,但是有關石墨炔體系電子輸運性質方面的研究卻鮮有報道。由于石墨炔中組成π電子的碳原子軌道成份不同,導致以往關于石墨烯結構的電子輸運性質的結論不能完全適用于石墨炔體系。因此,研究石墨炔體系的電子輸運性質,探索適用于研究sp2和sp雜化體系的電子輸運機制的理論具有非常重要的理論和應用價值。本論文中的三個工作就是基于石墨烯和石墨炔這兩種材料展開研究的。1. ZGNRs電極與石墨一炔子結構的電子輸運特性考慮到石墨一炔結構中包含sp2和sp兩種不同的雜化模式,它的電子輸運特性以及相關的物理機制問題研究的較少,因此探索適用于研究sp2和sp雜化體系的電子輸運機制的理論具有非常重要的理論和應用價值�？紤]到電極與中心分子的接觸電阻問題,我們選取了ZGNRs作為電極,這是由于它們都是二維的平面材料,并且ZGNRs具有邊緣態(tài),能夠在Fermi能級處產(chǎn)生較強的電子密度分布,這種奇特電子密度分布可能會對基于ZGNRs電子器件產(chǎn)生特殊的輸運特性。我們利用密度泛函理論和非平衡格林函數(shù)相結合的方法,研究了石墨一炔子結構與鋸齒形石墨烯電極構成的三明治結構的電荷輸運特性。主要考慮石墨一炔與電極鏈接不同位置以及電極的不同寬度對該體系電荷輸運特性的影響。我們發(fā)現(xiàn)：①無論石墨一炔與ZGNRs電極的鏈接位置如何,體系都會表現(xiàn)出半導體的電荷輸運特性,這是由石墨一炔體系的HOMO-LUMO gap決定的。②當石墨一炔鏈接在ZGNRs電極的邊緣位置處的電流要比石墨一炔鏈接在ZGNRs電極中心位置處的電流要大,并且隨著鏈接位置越來越靠近ZGNRs中心處,電流的大小會越來越小。③當整個體系關于xz中垂面鏡面對稱時,宇稱限制隧穿效應可以完全破壞掉體系的電子輸運特性,增大體系的開啟偏壓。在本研究工作中,我們著重對這三個現(xiàn)象的物理機制進行了系統(tǒng)性的探討。2.鋸齒形石墨一炔(ZGYRs)與ZGNRs異質結的整流特性研究傳統(tǒng)的整流觀點是要保證體系具有不對稱性,這樣就可能會產(chǎn)生整流現(xiàn)象,從而制造分子整流器件。我們利用密度泛函理論和非平衡格林函數(shù)相結合的方法計算了兩種不同的體系ZGYRs和ZGNRs構成異質結的整流特性,發(fā)現(xiàn)不對稱的體系不一定能夠產(chǎn)生整流現(xiàn)象。為了更深入的研究這個問題,我們對ZGYRs進行了氧原子的替換,我們發(fā)現(xiàn)未受氧原子替換的ZGYRs與ZGNRs形成的異質結沒有出現(xiàn)整流現(xiàn)象,這主要歸因于這兩個體系的能帶結構是關于Fermi能級對稱的,在正負偏壓的影響下,電流都是對稱性分布的。而當ZGYRs被氧原子替換后,體系的整流方法發(fā)生變化,替換不同位置,整流方向是不相同的。更重要的是,如果氧原子是對稱性替換掉ZGYRs的話,體系會表現(xiàn)出偏壓誘導的整流反轉現(xiàn)象。3.局域應力下的ZGNRs的電子輸運特性先前的工作者對ZGNRs的電荷輸運性質的研究發(fā)現(xiàn),偶數(shù)寬度的ZGNRs具有鏡面對稱性,因此它的電流大小會受到宇稱限制隧穿效應的影響,從而表現(xiàn)出電流抑制效應。我們設想如果偶數(shù)寬的ZGNRs的對稱性被部分的打破,是否還會出現(xiàn)電流抑制效應?基于這一點,我們利用密度泛函理論和非平衡格林函數(shù)相結合的方法,研究了ZGNRs在外加局域應力后的電荷輸運特性,主要考慮應力作用的不同范圍以及應力作用的大小對該體系電荷輸運性質的影響。研究發(fā)現(xiàn),在較小的局域應力作用下,體系的電子輸運特性基本上沒有發(fā)生變化,表現(xiàn)出與未受應力的體系相同的電流曲線特征-電流抑制現(xiàn)象。繼續(xù)增大應力后,一些體系在應力的作用下誘導產(chǎn)生了局域態(tài),并且我們發(fā)現(xiàn)局域態(tài)在零偏壓下阻礙的電子的傳輸。然而在有限偏壓下,應力誘導的局域態(tài)卻可以促進ZGNRs的電荷輸運行為,原因在于偏壓下的局域態(tài)破壞了ZGNRs電子密度分布的對稱性,從而打破了電流抑制效應。進一步研究發(fā)現(xiàn),應力作用的范圍不同也會明顯影響體系的電子輸運特性,應力作用在邊緣原子上對體系的電子輸運的改變小于應力作用在中心區(qū)域,并且體系原本的電子密度分布也對應力的響應不同,也會對體系電子輸運性質產(chǎn)生影響。
[Abstract]:With the development of electronic devices miniaturization, the size of traditional silicon based integrated circuits is shrinking. According to the prediction of the famous Moore's law, the size of silicon based devices will be further reduced to about 1 ~ 2nm. The size of this size has already entered the field of molecules and atoms, and can be formed by a single molecule. This idea is a natural concern in nanoscale circuits. It makes people feel happy that a single molecule can actually miniaturized the traditional silicon based microelectronic devices and can also show the functions of traditional silicon based devices. So, we use a single atom or use a single point. The way to build functional components in electronic circuits is called molecular electronics. All the time, people have been designing electronic devices to find and prepare molecular sizes for.1959. The American physicist Feynman published a famous speech "Plenty of Room at the Bottom" at the annual meeting of the Physics Society, which was put forward in molecules and atoms. The idea of building an electronic device in size and forming an integrated circuit had a profound effect at the time. The traditional idea was to establish macro systems and components and reduce their size, and Feynman's idea was to establish electronic devices directly from the atomic scale and then combine this electronic device into a circuit. Compared to the traditional silicon based semiconductor materials, the elements used as electronic devices have many great advantages, such as: (1) size dimension: small size of the molecule (1-10nm) and very high integration density, which has great advantages in energy consumption and utilization efficiency mask; Molecular lines with better transport properties can reduce the transition time of transistors and reduce operation time; (3) assembly and recognition: a nano scale self-assembly structure can be formed by special intermolecular interaction. Molecular recognition can be used to regulate electronic behavior, provide switching and perceptual functions on a single sub scale; (4) synthesis Modifiability: the selection of different combinations and structures can change the molecular transport properties of molecules; (5) new functions: some special types of molecules have different stable structures or isomers, which can not be realized in traditional solid materials. With the development of experimental techniques, various molecular electronic devices are available. In order to prepare it, this is due to these advanced equipment. Here we simply introduce several kinds. (1) the scanning electron microscope (STM) is one of the most common methods for preparing the atomic scale contact. The experimental method is to put the solution containing the target molecules on the metal substrate and use the STM probe on the surface of the metal substrate. With the expansion motion, the probe, the sandwich electrode structure of the target molecule and the metal substrate are formed, and the conductance values are measured. In addition, the atomic force microscope technique, the transmission electron microscope (TEM), mechanical and controllable breaking and so on. With the development of the experimental means, the theoretical method is constantly innovating and used for more precise expression. Charge or spin transport in molecular electronic devices. At present, the combination of density functional theory and nonequilibrium Green function method is the first choice to deal with molecular electron transport, and has been widely used in related studies, such as single molecular clusters, molecular lines, graphene, and nanotubes, etc. since 2004, Manchester United The study of graphene has attracted wide attention since the first single graphene was separated from Novoselov and Geim in the University. In this period, two-dimensional plane materials have become the hot spots. Based on the single graphene, two different directions are cut, and two kinds of materials can be obtained. The same kind of graphene nanoscale bands. Usually, the graphene nanometers can be divided into serrated graphene nanometers (ZGNRs) and armchair graphene nanoscale ribbons (AGNRs) according to the edge patterns. The research on ZGNRs is more prosperous. The previous researchers focused on the study of the electron transport of ZGNRs. In this case, a part of ZGNRs is linked to the ZGNRs electrode. In this case, the contact position effect of the ZGNRs fragment and the ZGNRs electrode is ignored. Furthermore, the ZGNRs has the edge state, which can produce a strong electron density distribution at the Fermi level. This peculiar electron density distribution may produce a special electronic density distribution based on the ZGNRs electronic devices. Therefore, the research based on ZGNRs is very important. In the study of SP2 hybrid graphene, people continue to explore the Homo heteromorphs with different kinds of hybrid forms of carbon. Because of the unique SP2 and SP hybrid properties of graphite alkynes, the research of graphite and acetylene in recent years has also been widely concerned, Hirsch It is believed that the new era of graphite acetylene is coming. Although scientists have done a lot of research on the structure, synthesis and properties of graphite alkynes, there are few reports about the electronic transport properties of graphite alkynes. The conclusion that the electronic transport properties of the graphene structure can not be fully applied to the graphite alkyne system. Therefore, it is very important to study the electronic transport properties of the graphite acetylene system and explore the theory and application value for the study of the electronic transport mechanism of SP2 and SP hybrid systems. The three work in this paper is based on graphene and stone. The electronic transport properties of.1. ZGNRs electrodes and graphite monyne structures studied by the two kinds of graphite alkynes are considered as two different hybrid modes of SP2 and SP in the graphite monyne structure. The electron transport properties and related physical mechanism problems are less studied. Therefore, the probe is suitable for the study of the electrons of the SP2 and SP hybrid systems. The theory of transport mechanism has a very important theoretical and practical value. Considering the contact resistance between the electrodes and the central molecules, we choose ZGNRs as the electrode, which is because they are two-dimensional plane materials, and ZGNRs has the edge state, which can produce strong electron density distribution at the Fermi level, this strange electron. Density distribution may produce special transport properties for ZGNRs based electronic devices. Using the method of density functional theory and nonequilibrium Green function, we study the charge transport properties of the sandwich structure composed of the graphite monyne structure and the sawtooth graphene electrode. The main consideration is that the graphite one acetylene is linked to the electrode. The influence of the different widths of the electrode and the electrode on the charge transport characteristics of the system. We found that: (1) the charge transport properties of the semiconductor, regardless of the location of the link between the graphite one and the ZGNRs electrode, are determined by the HOMO-LUMO gap of the Shi Moyi acetylene system. The current at the position of the set is larger than the graphite one at the center of the ZGNRs electrode. And as the link position gets closer to the center of the ZGNRs, the size of the current will become smaller and smaller. In this research work, we have focused on the physical mechanism of the three phenomena systematically to study the rectifying characteristics of the.2. sawtooth graphite one acetylene (ZGYRs) and the ZGNRs heterojunction. The traditional rectification viewpoint is to ensure that the system has asymmetry, which may produce rectifying phenomena and thus make the molecules. Rectifying devices. We use the density functional theory and the nonequilibrium Green function method to calculate the rectifying characteristics of two different systems, ZGYRs and ZGNRs, and find that the asymmetric system does not necessarily produce rectifying phenomena. In order to further study this problem, we have replaced the oxygen atom in ZGYRs. We found that the heterostructure formed by the unoxygen atom substitution of ZGYRs and ZGNRs has no rectifying phenomenon, which is mainly attributed to the band structure of the two systems is about the symmetry of the Fermi energy level, and the current is symmetrical under the influence of positive and negative bias. When the ZGYRs is replaced by oxygen atom, the rectification method of the system is changed and replaced. In different positions, the rectification direction is different. More importantly, if the oxygen atom is symmetric and replace the ZGYRs, the system will show the bias induced rectifying and reversal phenomenon, the electron transport properties of the ZGNRs under the local stress of.3., the previous workers on the charge transport quality of the ZGNRs found that the even number of ZGNRs has a mirror surface. We assume that if the symmetry of the even wide ZGNRs is partially broken, will there be a current suppression effect? Based on this, we use the combination of the density functional theory and the nonequilibrium Green function. The charge transport characteristics of ZGNRs after external stress are studied. The effect of the different range of stress and the size of stress on the charge transport properties of the system is mainly considered. It is found that the electronic transport properties of the system have not changed under the effect of small local stress, and show that the system is not subjected to the stress. The same current curve characteristic current suppression. After the stress continues to increase, some systems induce local states under the action of stress, and we find the transmission of electrons hindered by the local state under the zero bias. However, under the finite bias, the local state induced by the stress can promote the charge transport behavior of ZGNRs. It is that the local state under partial pressure destroys the symmetry of the distribution of ZGNRs electron density, thus breaking the effect of current suppression. Further studies have found that the range of the stress action will obviously affect the electronic transport properties of the system, and the change of the electron transport of the stress on the edge atom on the system is less than the stress in the central region, Moreover, the original electron density distribution also responds to the force response, which will also affect the electron transport properties of the system.
【學位授予單位】：山東大學
【學位級別】：博士
【學位授予年份】：2016
【分類號】：O469

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